The Taming of the Blue: River engineering along the Danube and its impact on the sedimentary record downstream of Vienna.

Since Roman times, the Danube River has been an important traffic way. Along the banks of this stream, some of the oldest cities of Central and Eastern Europe were built. Many conflicts were fought out in the Danube floodplains for hundreds of years with the aim of getting control over navigation, commercial centres, strongholds, and the surrounding territories.After a long period of wars and political crisis during the first half of the 19th century, the capital of the Habsburg Empire started a series of improvement measurements projected long before. Besides the drinking water supply and the demolition of the city's fortifications, the Danube River engineering was the most momentous building project in this period of industrialization. Thanks to new efficient machines and know-how developed by digging the Suez Canal, it was possible to corset the branched Danube River in a new and straightened bed, limited by bank reinforcements and dykes. A highly dynamic river system characterized by fast-changing flow rates, variable water levels, and migrating river branches was transformed in a well-defined channel.Great effort was made to achieve three goals: good navigability along this part of the Upper Danube, flood protection sufficient for a growing metropolis, and channelized waterways near to the urban centres for the supply of foods and wood and for the wastewater draining as well. The requirements of river engineering had various effects on the resulting fluvial dynamics; some of them were premeditated, others were unfavourable but accepted, and several were entirely unexpected.This study aims to analyse the evolving fluvial system and assess the role of human intervention by investigating sequences of flood events archived within levees in selected places downstream of Vienna. By combining sedimentological with historical methods, we seek to show the sedimentary evidence of a changing fluvial system responding to the increasing human impact during the last 200 years. Primarily, the channelizing of the Danube River resulted in increased flow rates and accelerated bed load transportation. These changes significantly impacted the volume of moved gravel within the riverbed and contributed to fluctuating water levels. As a result of the river straightening process, water levels often deviate from desired conditions – occasionally rising higher but more frequently being lower than optimal levels. Bank reinforcements hinder lateral water flow and erosion as well, resulting in the aggradation within the floodplain and loss of habitats.Since the 1950s, a chain of ten hydroelectric power plants has been built along the Austrian Danube blocking the transport of coarse bed load by barrages. The alternation of backwater areas upstream of the dams and fast-flowing stretches below results in a fractionated sedimentation of coarse bedload and fine sediment within different sections of the Danube. Extreme flood events can mobilize large amounts of such separate deposits, forming huge levees after the redeposition within the free-flowing section downstream of Vienna.